The fossil fuel energy crisis which is rapidly engulfing the civilized world is focusing attention on the wasteful practices of that world. One of those wasteful practices which should be eliminated resides in the method of operation of environmental conditioning systems such as heating and cooling units for residential and commercial dwellings.
The majority of environmental conditioning systems function by monitoring the temperature of a space to be heated or cooled by an automatic means which causes a heating or cooling unit to be activated when the controlled space temperature exceeds a predetermined value. For instance consider a typical residential heating unit which utilizes a fossil fuel burner in combination with a forced air system. When the temperature of the structure decreases below a predetermined minimum, the fossil fuel burner is ignited and air (or other heat transfer media) is forced over a heat exchanger in a duct system. After a brief period of time, the heat exchanger reaches a relatively high temperature and its ability to absorb heat decreases. This results in the discharge of significant quantities of surplus heat through the exhaust gas system or flue rather than being absorbed by the air or other cooling media passing on the other side of the heat exchanger. When the temperature in the controlled space exceeds a predetermined value, the fossil fuel burner is turned off. The forced air system continues to function and the residual heat in the heat exchanger is utilized to increase the heat in the control space above the turn-off value. During this portion of the cycle of operation, both sides of the heat exchanger are significantly cooler than the heat exchanger member and therefore some heat is radiated both up the flue (a second source of waste) and into the forced air system. The heat discharged through the flue is a waste of fossil fuels and is a direct function of the temperature differential between the heat exchange surfaces and the burner side of the heat exchange unit. That is, as the heat exchanger becomes hotter, it resists absorption of heat from the burner fire. This heat which is not absorbed is then discharged through the exhaust gas flue system and represents a significant loss in total system efficiency.
Prior art systems have been developed which attempt to minimize the above suggested losses. However, these attempts are relatively complex and costly, requiring an excessive amount of hardware and maintenance but offering relatively poor efficiency.
For instance, the Verden patent, U.S. Pat. No. 3,401,880 on "Compensated Temperature Control System" discloses a system which includes a temperature responsive device in a controlled space and a temperature responsive device in the heating plenum. The system functions to maintain plenum heat at predetermined levels by cycling the heating apparatus on and off at a duty cycle which is a function of the demands for heat of the control space. The system utilizes a time delay heat responsive element to control cycle durations and is not precise or adjustable. The purpose of this invention was to compensate the room thermostat so as to minimize overshoot and undershoot common to such devices and thereby maintain a more uniform dewlling temperature. It required multiple sensors and made no attempt to optimize cycle rate for efficiency.
Devices such as the Verden system place a relatively delicate switching means in an extremely hostile environment where it is subjected to a relatively large number of operational cycles, resulting in high failure rates. They moreover failed to provide proportional control by their dependence on a normal thermostatic sensor.
Another contemporary approach to an environmental temperature control system utilizing digital counters is exemplified by Weatherston in U.S. Pat. No. 3,677,335 on "Staged Heating And Cooling System". This patent teaches the concept of utilizing a digital system to control a heating and air conditioning system by sequentially stepping the temperature modifying unit through a predetermined series of steps calculated to cause a controlled space to reach a predetermined temperature. Systems such as this become increasingly inefficient as the environmental temperature outside of the controlled space varies or as heat losses from the controlled space varies due to structure usage. Theys respond slowly to changes in demand and are characterized by large temperature overshoot or undershoot during periods of changing temperature.